1. Field of the Invention
The present invention relates to a plastic thermistor and a thermosensitive device comprising the same. In particular, the present invention relates a plastic thermistor which is used in flexible temperature sensors or thermosensitive heaters such as electric heaters and the like, and a thermosensitive device comprising the plastic thermistor.
2. Prior Art
In general, a plastic thermistor is provided between a pair of electrodes, and used as a flexible linear temperature sensor or thermosensitive heater.
Conventional plastic thermistors comprise a composition containing polyamide such as Nylon 12 or modified Nylon 11 which is disclosed in JP-A-55-100693 (trade name: RILSAN N NYLON available from ATO-CHIMIE). The plastic thermistors function as temperature sensors by utilizing temperature change of their electrostatic capacity, resistance or impedance.
JP-B-60-48081 discloses a polyamide composition containing a phosphite ester as an improver for heat deterioration, and JP-A-64-30203 discloses an ionically conductive thermosensitive composition containing a copper inactivation agent and a phenolic antioxidant.
However, it is difficult to use Nylon 12 practically, since it has low moisture absorbability, but its thermosensing characteristics vary widely depending on the humidity when it is used as the temperature sensor.
The modified polyamide which is disclosed in JP-A-55-100693 has poor heat stability, and low temperature sensing properties since it has less temperature dependence of impedance.
It is proposed to compound a polycondensate of a phenol compound with an aldehyde for the improvement of moisture resistance and thermosensitivity, as disclosed in JP-B-3-50401. Furthermore, JP-A-58-215449 proposes a polyamide composition comprising 0.02 to 5 wt. % of zinc iodide for increasing the temperature dependence of impedance. However, the former composition has low temperature dependence of impedance, while the latter composition has improved temperature dependence of the initial impedance but it still has insufficient heat stability of an impedance-temperature curve like the former composition.